What's New in Nanotechnology?

The image illustrates how proteins (copper-colored coils) modified with polyhistidine-tags (green diamonds) can be attached to nanoparticles (red circle). Credit: SUNY University at Buffalo, Jonathan Lovell.

Fastening protein-based medical treatments to nanoparticles isn’t easy. With arduous chemistry, scientists can do it. But the fragile binding that holds them together often separates. This problem, which has limited how doctors can use proteins to treat serious disease, may soon change. University at Buffalo researchers have discovered a way to easily and effectively fasten proteins to nanoparticles – essentially an arranged marriage – by simply mixing them together. While in its infancy, the biotechnology model already has shown promise for developing an HIV vaccine and as a way to target cancer cells.

To create the biotechnology, the researchers use nanoparticles made of chlorophyll (a natural pigment), phospholipid (a fat similar to vegetable oil) and cobalt (a metal often used to prepare magnetic, water-resistant and high-strength alloys). The proteins, meanwhile, are modified with a chain of amino acids called a polyhistidine-tag. Uncommon in medicine, polyhistidine-tags are used extensively in protein research. Next, the researchers mixed the modified proteins and nanoparticles in water. There, one end of the protein embeds into the nanoparticle’s outer layer while the rest of it sticks out like a tentacle. To test the new binding model’s usefulness, the researchers added to it an adjuvant, which is an immunological agent used to enhance the efficacy of vaccines and drug treatments. The results were impressive. The three parts – adjuvant, protein and nanoparticle – worked together to stimulate an immune response against HIV.

New and improved solar panels could result from the discovery of a new liquid crystal material, making printable organic solar cells better performing. University of Melbourne researchers say their discovery of the highly sought-after ‘nematic liquid crystals’ can now lead to vastly improved organic solar cell performance. Dr David Jones of the University’s School of Chemistry and Bio 21 Institute, said these cells will be easier to manufacture, with the new crystals now able to work in cells that are double in thickness on the previous limit of 200 nanometers. “We have improved the performance of this type of solar cell from around 8 per cent efficient to 9.3 per cent, finally approaching the international benchmark of 10 per cent.” It means that consumers can look forward to more competitive pricing in the solar energy sector, and according to Dr Jones, the discovery is a shot-in-the-arm for the whole organic materials sector. “The discovery is a step forward for the wider commercialization of printed organic solar cells. But more than this, could aid in the development of new materials with improved performance such as LCD screens.” Uptake of the current generation of organic solar cells has lagged behind more widespread silicon-based models, due to their comparative lack of performance even with a simplified construction via large printers.

Graphene nanoribbons can be enticed to form favorable “reconstructed” edges by pulling them apart with the right force and at the right temperature, according to researchers at Rice University. The illustration shows the crack at the edge that begins the formation of five- and seven-atom pair under the right conditions. (Credit: ZiAng Zhang/Rice University)

Theoretical physicists at Rice University are living on the edge as they study the astounding properties of graphene. In a new study, they figure out how researchers can fracture graphene nanoribbons to get the edges they need for applications. New research by Rice physicist Boris Yakobson and his colleagues shows it should be possible to control the edge properties of graphene nanoribbons by controlling the conditions under which the nanoribbons are pulled apart. The way atoms line up along the edge of a ribbon of graphene — the atom-thick form of carbon — controls whether it’s metallic or semiconducting. Current passes through metallic graphene unhindered, but semiconductors allow a measure of control over those electrons. Since modern electronics are all about control, semiconducting graphene (and semiconducting two-dimensional materials in general) are of great interest to scientists and industry working to shrink electronics for applications. In the work, the Rice team used sophisticated computer modeling to show it’s possible to rip nanoribbons and get graphene with either pristine zigzag edges or what are called reconstructed zigzags.

Perfect graphene looks like chicken wire, with each six-atom unit forming a hexagon. The edges of pristine zigzags look like this: /\/\/\/\/\/\/\/\. Turning the hexagons 30 degrees makes the edges “armchairs,” with flat tops and bottoms held together by the diagonals. The electronic properties of the edges are known to vary from metallic to semiconducting, depending on the ribbon’s width. “Reconstructed” refers to the process by which atoms in graphene are enticed to shift around to form connected rings of five and seven atoms. The Rice calculations determined reconstructed zigzags are the most stable, a desirable quality for manufacturers. All that is great, but one still has to know how to make them. “Making graphene-based nano devices by mechanical fracture sounds attractive, but it wouldn’t make sense until we know how to get the right types of edges — and now we do,” said ZiAng Zhang, a Rice graduate student.

Two-dimensional (2D) materials* such as molybdenum-disulfide (MoS2) are attracting much attention for future electronic and photonic applications ranging from high-performance computing to flexible and pervasive sensors and optoelectronics. But in order for their promise to be realized, scientists need to understand how the performance of devices made with 2D materials is affected by different kinds of metal electrical contacts.

Researchers in the National Institute of Standards and Technology (NIST) Physical Measurement Laboratory's Semiconductor & Dimensional Metrology Division, in collaboration with researchers from George Mason University, compared silver and titanium contacts on MoS2 transistors to determine the influence of the metal–MoS2 interface. A sophisticated suite of measurements (Raman spectroscopy, scanning electron microscopy and atomic force microscopy) was used to characterize the surface morphology and the interface between MoS2 and the metals, the properties of which affect the device behavior. It was found that silver provides a much better electrical contact to MoS2 than the widely used titanium, with the silver-contact devices having 60 times higher current when the device is in the "on" state. These results are another step towards the advanced manufacture of high-value products based on 2D materials.

Spherical gold particles are able to ‘drill’ a nano-diameter tunnel in ceramic material when heated. This is an easy and attractive way to equip chips with nanopores for DNA analysis, for example. Researcher Lennart de Vreede of the University of Twente applied a large number of microscopic discs of gold on a surface of silicon dioxide. When heated up for several hours, the gold is moving into the material, perpendicular to the surface, like nanometer-sized spheres. Nine hours of heating gives a tunnel of 800 nanometers in length, for example, and a diameter of 25 nanometer: these results can normally only be acieved by using complex processes. The gold can even fully move through the material. All nanotunnels together then form a sieve. Leaving the tunnel closed at one end, leaves open the possibility of creating molds for nano structures. Once heated to close to their melting point, the gold discs – diameter one micron -, don’t spread out over the surface, but they form spheres. They push away the siliciumdioxide, causing a circular ‘ridge’, a tiny dam. While moving into the silicondioxide, the ball gets smaller: it evaporates and there is a continuos movement of silicondioxide.

In DNA-sequencing applications, De Vreede sees applications for this new fabrication technology. In that case, a DNA-string is pulled through one of these nano-channels, after which the building blocks of DNA, the nucleotides, can be analysed subsequently. Furthermore, De Vreede expects the ‘gold method’ to be applicable to other ceramic materials as well. His recent experiments on silicium nitride indicate that. Research has been done in the BIOS Lab-on-a-chip group, part of two research institutes of the University of Twente: the MESA+ Institute for Nanotechnology and the MIRA Institute for Biomedical Technology and Technical Medicine.

Scientists at the U.S. Department of Energy’s Oak Ridge National Laboratory have used advanced microscopy to carve out nanoscale designs on the surface of a new class of ionic polymer materials for the first time. The study provides new evidence that atomic force microscopy, or AFM, could be used to precisely fabricate materials needed for increasingly smaller devices. Polymerized ionic liquids have potential applications in technologies such as lithium batteries, transistors and solar cells because of their high ionic conductivity and unique structure. But many aspects of the recently discovered materials are still not well understood. When ORNL researchers used an atomic force microscope to begin characterizing the properties of polymerized ionic liquid thin films, the experiment yielded some surprising results.

“We were expecting to measure ionic conductivity, and instead we found that we were forming holes on the surface,” said ORNL’s Vera Bocharova, corresponding author on the study published in Advanced Functional Materials. “Then we started to think about how this might have great applications in nanofabrication.” Nanolithography is the dominant technique used by industry for nanofabrication, but its size limitations are leading researchers to explore other methods such as AFM. “This study is part of our search for alternative methods and materials that can be used to create smaller sized objects,” Bocharova said. “For example, our technique might be interesting for the miniaturization of semiconductor technology.”

Chuck Black of the Center for Functional Nanomaterials displays a nanotextured square of silicon on top of an ordinary silicon wafer. The nanotextured surface is completely antireflective and could boost the production of solar energy from silicon solar cells. (Image Credit: BNL)

Reducing the amount of sunlight that bounces off the surface of solar cells helps maximize the conversion of the sun's rays to electricity, so manufacturers use coatings to cut down on reflections. Now scientists at the U.S. Department of Energy's Brookhaven National Laboratory show that etching a nanoscale texture onto the silicon material itself creates an antireflective surface that works as well as state-of-the-art thin-film multilayer coatings. Their method has potential for streamlining silicon solar cell production and reducing manufacturing costs. The approach may find additional applications in reducing glare from windows, providing radar camouflage for military equipment, and increasing the brightness of light-emitting diodes. "For antireflection applications, the idea is to prevent light or radio waves from bouncing at interfaces between materials," said physicist Charles Black, who led the research at Brookhaven Lab's Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility.

Preventing reflections requires controlling an abrupt change in "refractive index," a property that affects how waves such as light propagate through a material. This occurs at the interface where two materials with very different refractive indices meet, for example at the interface between air and silicon. Adding a coating with an intermediate refractive index at the interface eases the transition between materials and reduces the reflection, Black explained. "The issue with using such coatings for solar cells," he said, "is that we'd prefer to fully capture every color of the light spectrum within the device, and we'd like to capture the light irrespective of the direction it comes from. But each color of light couples best with a different antireflection coating, and each coating is optimized for light coming from a particular direction. So you deal with these issues by using multiple antireflection layers. We were interested in looking for a better way." For inspiration, the scientists turned to a well-known example of an antireflective surface in nature, the eyes of common moths. The surfaces of their compound eyes have textured patterns made of many tiny "posts," each smaller than the wavelengths of light. This textured surface improves moths' nighttime vision, and also prevents the "deer in the headlights" reflecting glow that might allow predators to detect them.

Schematic of an infrared photodetector with graphene as its active element (Image Credit: AMO GmbH)

Infrared photodetectors in communications systems have traditionally been built as discrete devices connected to the optical fibre carrying the signal, and an electronic circuit for processing the received data. An improvement on this arrangement would be to integrate the detector and electronics on a single chip. This would substantially reduce the device footprint and fabrication cost. The maximum data rate achieved with a state-of-the-art germanium detector fabricated using the standard silicon-based CMOS production system for integrated circuits is 40 gigabits per second. However, the performance of such photodetectors is limited by the material properties, and is less than optimal, owing to silicon’s vanishing light absorption at the wavelengths used. This is driving the search for new and better materials, and graphene is considered a promising candidate.

In a paper recently published in the journal ACS Photonics, Daniel Schall and a team based at AMO in Aachen, and Alcatel-Lucent Bell Labs in Stuttgart, demonstrated photodetectors based on wafer-scale graphene. The devices are capable of recording data at up to 50 gigabits per second, and display excellent signal integrity. Study leader Daniel Schall is a 32-year-old electrical engineer who has been with AMO since 2009, and is currently working toward a PhD at RWTH Aachen University. His work on graphene is supported by the European Commission through the Graphene Flagship.

One of the reasons solar cells are not used more widely is cost — the materials used to make them most efficient are expensive. Engineers are exploring ways to print solar cells from inks, but the devices don’t work as well. Elijah Thimsen, PhD, assistant professor of energy, environmental & chemical engineering in the School of Engineering & Applied Science at Washington University in St. Louis, and a team of engineers at the University of Minnesota have developed a technique to increase the performance and electrical conductivity of thin films that make up these materials using nanotechnology. Transparent conductors are thin films, which are are simply ultrathin layers of materials deposited on a surface that allow light to pass through and conduct electricity, a process in which electrons flow through a system. Thimsen and his team found by changing the structure of a thin film made of zinc oxide nanoparticles, electrons no longer flowed through the system in a conventional way, but hopped from place to place by a process called tunneling.

The team measured the electronic properties of a thin film made of zinc oxide nanoparticles before and after coating its surface with aluminum oxide. Both the zinc oxide nanoparticles and aluminum oxide are electronic insulators, so only a tiny amount of electricity flows through them. However, when these insulators were combined, the researchers got a surprising result. “The new composite became highly conductive,” Thimsen said. “The composite exhibits fundamentally different behavior than the parent compounds. We found that by controlling the structure of the material, you can control the mechanism by which electrons are transported.” Because the reason behind this is not well understood, Thimsen and the team plan to continue to work to understand the relationship between the structure of the nanoparticle film and the electron transport mechanism, he said.

A schematic shows the process developed by Rice University scientists to make vertical microsupercapacitors with laser-induced graphene. The flexible devices show potential for use in wearable and next-generation electronics. Click on the image for a larger version. Image Credit: Rice University/Courtesy of the Tour Group.

Rice University scientists advanced their recent development of laser-induced graphene (LIG) by producing and testing stacked, three-dimensional supercapacitors, energy-storage devices that are important for portable, flexible electronics. The Rice lab of chemist James Tour discovered last year that firing a laser at an inexpensive polymer burned off other elements and left a film of porous graphene, the much-studied atom-thick lattice of carbon. The researchers viewed the porous, conductive material as a perfect electrode for supercapacitors or electronic circuits. An electron microscope image shows the cross section of laser-induced graphene burned into both sides of a polyimide substrate. The flexible material created at Rice University has the potential for use in electronics or for energy storage. Click on the image for a larger version. To prove it, members of the Tour group have since extended their work to make vertically aligned supercapacitors with laser-induced graphene on both sides of a polymer sheet. The sections are then stacked with solid electrolytes in between for a multilayer sandwich with multiple microsupercapacitors. The flexible stacks show excellent energy-storage capacity and power potential and can be scaled up for commercial applications. LIG can be made in air at ambient temperature, perhaps in industrial quantities through roll-to-roll processes, Tour said.